In general, in an ultrasonographic equipment intended to three-dimensionally display an intravital tissue aspect, the following structure has been known (for example, refer to Japanese Patent Document 1 as below). In the structure, as an ultrasonic probe for capturing three-dimensional echo data, an ultrasonic transducer unit for scanning an ultrasonic beam performs mechanical oscillation scanning in the direction crossing the beam scanning direction. FIG. 12 is an explanation drawing showing a scanning state thereof. In FIG. 12, an ultrasonic transducer unit 1 in an unshown probe includes a plurality of ultrasonic transducer elements which are arranged to align in the horizontal direction X in relation to the surface of a test body (convex in FIG. 12). The ultrasonic transducer unit 1 transmits and receives ultrasonic waves in the depth direction Y of the test body and two-dimensionally scans the X-Y plane. In addition, the ultrasonic transducer unit 1 is oscillated in the direction Z orthogonal to the X-Y plane and scans the oscillation direction Z.
In the foregoing ultrasonic probe, ultrasonic beam scanning (hereinafter referred to as main cross section scanning) and oscillation scanning are concurrently performed. Thereby, it becomes possible to acquire echo data from intravital tissues corresponding to a cross line of the both scanned faces which move every second, that is, acquire three-dimensional echo data. The acquired three-dimensional echo data is provided with processing of structuring a three-dimensional image, that is, processing of converting the three-dimensional echo data to image data from a virtual view point. Then, the image data is displayed by a display method to show the image in the plane as if the image is three-dimensional. Otherwise, a given cross section is displayed.
In structuring a three-dimensional image, it is necessary that direction components in the three-dimensional space of each echo data are known. In such an ultrasonographic equipment using the ultrasonic probe which performs mechanical oscillation scanning, the ultrasonic transducer unit is controlled to be oscillated so that an ideal oscillation angular velocity W in relation to time of the ultrasonic transducer unit results in profile 21 as shown in FIG. 13A, that is, so that an ideal oscillation angle θ in relation to time of the ultrasonic transducer unit results in profile 22 as shown in FIG. 13B. Further, as echo data used for structuring the three-dimensional image, the echo data obtained during period from t1 to t2 when the oscillation angular velocity W is relatively constant (W=w1). Furthermore, the three-dimensional image is structured on the assumption that by performing main cross section scanning at regular time intervals, the main cross section scanned face is a plane and each main cross section is conformal.
In general, for the purpose of improving a three-dimensional echo data acquisition rate per unit time, oscillation is controlled so that the following formula is established.w2=(−1)*w1On that basis, echo data is acquired during outward and homeward oscillation periods of outward period from t1 to t2 when W is w1 and homeward period t4 to t5 when W is w2. Here, as shown in FIG. 14, when a main cross section scanned face 51 indicated in full line is formed by setting a main cross section scanning direction 53 in an oscillation scanning outward route 56 and the same main cross section scanning direction 53 is used in an oscillation scanning homeward route 55, a main cross section scanned face 52 as indicated in dotted line is created in the oscillation homeward route, that is, discrepancy of scanned face angles is generated between the oscillation outward route and the oscillation homeward route. Therefore, the following method has been proposed (for example, refer to Patent document 2 as below). In the method, a main cross section scanning direction 54 which is the direction opposite to the direction 53 of the outward route is set in the oscillation homeward route. Thereby, discrepancy of scanned face angles between the oscillation outward route and the oscillation homeward route is modified.    Patent document 1: Japanese Patent Application Publication No. 3-184532 (FIG. 2)    Patent document 2: Japanese Patent Application Publication No. 2001-70301 (FIG. 3)
In recent years, a three-dimensional image by an ultrasonographic equipment is used not only for observing an intravital tissue aspect, but also for puncturing a test body while monitoring the three-dimensional image and a guideline, or for measuring a distance, an angle, an area, a volume and the like of organs, tumors, fetuses and the like. Thus, usability thereof has been increased. To meet such medical needs, it is essential that three-dimensional images provided by ultrasonographic equipment are structured more precisely, that is, are structured in a spatially accurate position than ever before.
However, in the foregoing existing ultrasonographic equipment, the three-dimensional image is structured on the assumption that the main cross section scanned face used for structuring the three-dimensional image is a plane, on the presupposition that the ultrasonic transducer unit performs oscillation scanning at constant angular velocity during the period of acquiring the three-dimensional echo data. In general, in mechanical oscillation scanning using a motor, so-called feedback control is used. In the feedback control, a motor impressed voltage or a current in next time is determined by a current oscillation scanning angle or a current oscillation scanning angular velocity, or the both thereof. However, even when the feedback control is performed highly precisely, it is not possible to perform oscillation scanning at a completely constant angular velocity. That is, it is not the fact that a main cross section scanned face of actually acquired echo data is not formed of a complete plane. Each main cross section is formed of a curved face to some extent. In result, in the three-dimensional image structured on the assumption that each main cross section scanned face is a plane, there are problems that the three-dimensional image is distorted or misaligned according to the degree of the curved face, or the image is swung according to oscillation reciprocation. In result, there is the possibility that puncture is performed in the direction shifted from the direction expected by an operator, or measurement errors in a distance, an angle, an area, a volume and the like are large.